Conventional methods for roughening aluminum materials or conventional methods for pearskin-finishing of aluminum materials include blasting, scratching, anodic electrolysis, wet etching, etc. Unfortunately, such conventional methods have problems such as large-scale facilities, high cost, high-temperature treatment, and large etching amount, and also have limitations on the quality, size, or other characteristics of the aluminum material capable of being treated, depending on conditions.
Patent Document 1 discloses a method that includes treating an aluminum surface with a fluoride ion-containing composition to make the surface porous. Unfortunately, stable aluminum fluoride is dissolved in the liquid waste after the treatment of aluminum with such a composition, because the composition contains fluoride ions. Therefore, the liquid waste treatment is complicated, so that it is difficult to reduce the cost of the treatment.
Patent Document 2 discloses a method for decoration by chemical pearskin-finishing of an aluminum surface with a chemical treating liquid containing an alkali source, amphoteric metal ions, nitrate ions, and sodium gluconate. Unfortunately, in the method disclosed in Patent Document 2, the treatment is performed at a high temperature of 50° C. or more, so that the amount of etching is large and that the roughened shape for decoration is not fine.